1. Field of the Invention
Aspects of the invention relate to signal transmission devices that perform simultaneous bi-directional communication between a master circuit and a slave circuit that are connected with isolation by a transformer.
2. Description of the Related Art
A switching power supply for controlling an electric power to be supplied to a load such as a motor has, for example, a construction as shown in FIG. 23 comprising a first and second switching elements Q1 and Q2 connected in series to compose a half-bridge circuit that are alternately ON/OFF-controlled to switch an input voltage E. The switching elements Q1 and Q2 can be a high-voltage IGBT or MOS-FET. The voltage VS at the series-connection point between the switching elements Q1 and Q2 is delivered to the load, which is not shown in the figure.
A switching control device ON/OFF-controls alternately the first and second switching elements Q1 and Q2 composing the main body of the switching power supply. The switching control device is provided with a microprocessor (MPU) 1 that generates control signals HIN and LIN for ON-driving the switching elements Q1 and Q2 alternately. A switching control circuit 3 generates driving control signals Hdrv and Ldrv for the switching elements Q1 and Q2 according to the signals XdrvH and XdrvL from an input buffer (BUF) 2 based on the control signals HIN and LIN.
A high side driver circuit 4 ON/OFF-drives the first switching element Q1 according to the driving control signal Hdrv, and a low side driver circuit 5 ON/OFF-drives the second switching element Q2 according to the driving control signal Wry.
The second switching element Q2 operates on a reference potential of the ground potential GND, while the first switching element Q1 operates on a reference potential of the voltage VS at the series connection point between the first switching element Q1 and the second switching element Q2, the voltage VS being the output voltage to the load. As a consequence, the high side driver circuit 4 for driving the first switching element Q1 is also operated on the reference potential of the voltage VS.
Consequently, in the path for transmitting the driving control signal Hdrv provided is a level shift circuit that level-shifts the output of the switching control circuit 3 and delivers to the high side driver circuit 4. Alternatively in place of the level shift circuit, an isolation circuit 6 is provided that electrically isolates the high side driver circuit 4 from the switching control circuit 3 and delivers the output of the switching control circuit 3 to the high side driver circuit 4. The isolation circuit 6 can be composed mainly of a coreless micro-transformer.
Such a type of isolation circuit 6 comprises, as shown in FIG. 24, two micro-transformers MT1 and MT2 for a set signal and a reset signal, a transmitting circuit TX connected to the primary sides of the micro-transformers MT1 and MT2, and a receiving circuit RX connected to the secondary sides of the micro-transformers MT1 and MT2. As shown by the operation waveforms in FIG. 25, a set signal SET-TX generated at the timing of leading edge of a control signal DATA-IN delivered to the transmitting circuit TX is given to the primary side of the micro-transformer MT1 for a set signal, and a set signal SET-RX is obtained from the secondary side of the micro-transformer MT1. A reset signal RES-TX generated at the timing of falling edge of the control signal DATA-IN is given to the primary side of the micro-transformer MT2 for a reset signal, and a reset signal RES-RX is obtained from the secondary side of the micro-transformer MT2. From the set signal SET-RX and the reset signal RES-RX, the control signal DATA-IN is demodulated to obtain an output signal DATA-OUT. This type of isolation circuit is disclosed in U.S. Pat. No. 7,692,444 (also referred to herein as “Patent Document 1”).
An isolation circuit can be composed using a single micro-transformer MT as shown in FIG. 26 and FIG. 28. Operation waveforms of the isolation circuit of FIG. 26 are shown in FIG. 27, in which the number of pulses of the set signal SET-TX generated at the timing of leading edge of the control signal DATA-IN delivered to the transmitting circuit TX and given to the primary side of the micro-transformer MT is different from the number of pulses of the reset signal RES-TX generated at the timing of falling edge of the control signal DATA-IN. Discrimination between the received set signal SET-RX and the reset signal RES-RX is performed detecting the number of pulses of the SET-RX and the RES-RX obtained at the secondary side of the micro-transformer MT. From the discriminated set signal SET-RX and reset signal RES-RX, the control signal DATA-IN is demodulated to obtain an output signal DATA-OUT. This type of isolation circuit is disclosed in U.S. Pat. No. 7,920,010 (also referred to herein as “Patent Document 2”).
Operation waveforms of the isolation circuit of FIG. 28 are shown in FIG. 29, in which the modulation frequency of the signal given to the primary side of the micro-transformer MT during a period of an H level of the control signal DATA-IN given to the transmitting circuit TX is made different from the modulation frequency during a period of an L level of the control signal DATA-IN. From discrimination of the modulated frequency of the signal obtained at the secondary side of the micro-transformer MT, the control signal DATA-IN is demodulated to obtain an output signal DATA-OUT. This type of isolation circuit is disclosed in U.S. Pat. No. 7,719,305 (also referred to herein as “Patent Document 3”).
The switching power supply shown in FIG. 23 performs control for protecting the switching elements Q1 and Q2 in which any abnormality occurred in the switching element Q1 or Q2 is detected and communicated from the side of the driver circuit 4 or 5 to the switching control circuit 3. The abnormality that possibly occurs in the switching element Q1 or Q2 includes, for example, overcurrent through the switching element Q1 or Q2, abnormal heating in the switching element, and abnormal drop of the output voltage VS due to load short-circuit.
The isolation circuits disclosed in Patent Documents 1, 2, and 3 however, are only provided with a unidirectional signal transmission capability that transmits the control signal Hdrv (or DATA-IN) only in the direction from the switching control circuit 3 to the driver circuit 4. In order to communicate the abnormality information (an alarm signal) mentioned above from the driving circuit 4 to the switching control circuit 3, another isolation circuit is required capable of signal transmission in the opposite direction. Thus, the isolation circuit inevitably becomes complicated.